Free thyroxine levels in hospitalized newborns: depressed levels in critical, nonthyroidal illness.

Thyroxine and triiodothyronine levels are lower in preterm infants than in term infants. Until a distinction is made among hypothyroidism, prematurity, and illness as the cause, the necessity of treatment is unclear. Thyroxine, triiodothyronine, free thyroxine, and thyroid-stimulating hormone were measured weekly in 75 patients of 25 to 42 weeks gestational age. The range of thyroxine values among hospitalized term and preterm infants was lower than has been previously reported for healthy term newborns or infants. Thyroxine and triiodothyronine levels were consistently lower in preterm infants, but there were only minor differences between term and preterm infants' free thyroxine levels, which were both stable over time. Mean (SD) free thyroxine levels by week were 0.0259 (0.0079), 0.0259 (0.0057), 0.0201 (0.0066), 0.0274 (0.0079), 0.0241 (0.0023) nmol/L for term infants, and 0.0215 (0.0072), 0.0223 (0.0056), 0.0212 (0.0066), 0.0186 (0.0064), and 0.0201 (0.0021) nmol/L for preterm infants at weeks 1, 2, 3, 4, and greater than 4, respectively. Maturity was the best predictor of thyroxine or triiodothyronine levels, but illness was the clinical variable most highly correlated with free thyroxine. All 71 surviving infants were euthyroid at discharge. The free thyroxine measurement results suggest that hypothyroxinemia is not hypothyroidism but is a result of a combination of reduced protein-bound thyroxine as well as nonthyroidal illness.